1. Technical Field of the Invention
The present invention relates to a method for forming an oxide film by depositing an oxide on a substrate by sputtering, and is particularly suited to forming an insulating layer in an electronic device exemplified by ICs or thin film magnetic heads.
In addition, the present invention relates to an electronic device in which the specific element formation layer is deposited on the insulating layer.
2. Background Information
Electronic devices, such as ICs and thin film magnetic heads, support Japan's industries. Every day new applications and new structures are developed.
There are many types of electronic devices. Most of these devices form an insulating layer of metal oxide film on a silicon wafer or a ceramic substrate on which a particular element formation layer is deposited.
In a thin film magnetic head, for example, an Al.sub.2 O.sub.3 -TiO system ceramic is the substrate, and a metal oxide thin film of silicon dioxide or aluminum oxide (Al.sub.2 O.sub.3, alumina) is formed on this substrate. This oxide film functions as the insulating layer.
These oxide films are usually formed by bias sputtering (Unexamined Japanese Patent No. 6-49637, Japanese Laid Open Patent No. 7-56070).
Bias sputtering is one type of sputtering. In addition to applying a negative voltage or a high-frequency voltage, a bias voltage is also applied to the substrate that is the deposition target.
Next, bias sputtering is described in detail.
For example, the metal oxide film is deposited by bias sputtering by using a bias sputtering system 100 like the one shown in FIG. 4.
In the bias sputtering system 100, the electrode called the target 102 is positioned opposite the substrate holder 103 in the airtight chamber 101. The target 102 and the substrate holder 103 are connected to the high-frequency power source (RF power source) 107 and the high-frequency power source (RF power source for biasing) 108 through the matching circuits 105 and 106, respectively.
The pressure reducing vacuum pump 110 and the gas supply source 111 for plasma generation are connected to the chamber 101.
In the bias sputtering system 100, the target 102 is a metal oxide film material made of aluminum oxide.
The substrate 1 that is the deposition target is mounted in the substrate holder 103.
The chamber 101 has an environment of a plasma generation gas (i.e., Ar, mixed gases of Ar and O.sub.2 or N.sub.2 in reactive sputtering) and applies the high-frequency voltage to the target 102 and the bias voltage to the substrate holder 103.
This generates a glow discharge plasma inside the chamber 101. Ar+ ions are attracted to the negative voltage of the target 102 and collide with the target 102.
The result is the metal oxides in the surface of the target 102 are sputtered. Then, the sputtered metal oxides adhere to the substrate 1 in the substrate holder 103.
In bias sputtering, because the bias voltage is also applied to the substrate 1, the metal oxides sputtered from the target 102 are attracted to the negative potential of the substrate 1, ion collisions occur with the metal oxides that already adhere to the substrate 1, and the metal oxides are released again. The result is the smoothing of the metal oxides on the substrate 1.
After the metal oxide is deposited in this manner on the substrate 1, it is chemically polished by alkali colloidal silica or colloidal alumina to further smooth the surface of the substrate 1. Then a magnetic film, a gap layer, and a conducting coil layer are deposited in layers on this substrate to fabricate the desired thin film magnetic head.
The methods for forming oxide film employing conventional technologies have spread widely as methods for fabricating the insulating layer, such as a thin film magnetic head, and produce a substrate 1 with a fairly smooth surface.
However, the methods described above that use conventional technologies have limits in the degree of smoothness of the substrate surface. In the end, unfortunately, some microscopic pits (concavities) remain on the surface.
In other words, even when the methods used previously for forming oxide film polish the substrate surface or control the film deposition conditions, such as the sputter gas flow, gas pressure, voltage applied on the target side, and substrate voltage, microscopic pits develop on the surface of the oxide film.
The reasons are explained next.
FIG. 5 is a diagram showing the process in the method for forming the oxide film using conventional technologies and shows the states entered when forming the oxide film on the substrate 1.
In a thin film magnetic head, a ceramic such as Al.sub.2 O.sub.3 -TiO is used as the substrate 1. Since the ceramic is a sintered material, many microscopic dents 2 similar those shown in FIG. 5(a) exist on the surface of the substrate 1.
When the metal oxide film 3 is deposited on the substrate 1 by bias sputtering as described above, the film surface will be uneven after deposition as shown in FIG. 5(b).
As described above, in order to further smooth the surface, the surface is chemically polished by an alkali colloidal silica or colloidal alumina. However, since the film growth differs for the metal oxide film 3 at the dents 2 in the substrate 1 and for the metal oxide film 3 at other places, the film quality exhibits small differences. In other words, the metal oxide film 3 on the dents 2 exhibits slight differences from other parts in the mixture amount of Ar or O.sub.2, and the fineness of the film.
Chemical polishing to smooth the film surface creates differences in the film quality, and differences develop in the etching speed at the metal oxide film 3 on the dents 2 and at other locations. Consequently, microscopic pits 5 develop on the metal oxide film 3 after chemical polishing.
Since the substrate surface is etched by the ions when film deposition begins in bias sputtering due to the negative voltage of the substrate side, the dents in the interface between the substrate 1 and the metal oxide film 3 further increase and are believed to be one cause of the pits 5.
When depositing magnetic film on a metal oxide film 3 having these kinds of pits 5, the pits 5 degrade the magnetic film characteristics or lower the yield of heads.
If a 1,000 .ANG. or smaller magnetic film is deposited for use in an MR head, in particular, the effect of the pits increases so the degradation of the magnetic film characteristics and the reduced yield of heads cannot be ignored.
In order to eliminate the microscopic pits 5 on the metal oxide film 3, efforts should concentrate on eliminating the dents 2 in the substrate 1. However, this technique is naturally limited to using a sintered material as the substrate 1.
Although the metal oxide film can be formed by methods other than bias sputtering, these methods have other problems such as requiring a long time to form the metal oxide film or a low film density. In practice, bias sputtering must be used.
Since the microscopic pits 5 on the metal oxide film 3 are difficult to eliminate in the conventional methods, new methods were desired by industry.
The present invention focuses on the problems described above which are found in the conventional technology and develops a method for forming oxide film that does not produce pits on its surface.